Self-healing link switched ring for ATM traffic

ABSTRACT

The invention provides for a method for transporting a SONET formatted asynchronous transfer mode (ATM) signal and/or a synchronous transfer mode (STM) signal on a line switched ring over a unidirectional path. The SONET formatted ATM signal comprises cells mapped into a STS-Mc or m×STS-1s while the STM signal comprises STS-1s/VTS mapped STS-W. A unidirectional line switched ring is provided for transporting the STM STS-W using a unidirectional path switched protection protocol and the ATM STS-Mc using a unidirectional line switched protection protocol. A ring node comprises input and output ring interfaces, an STS management block, an ATM cell management block, and a non-ATM payload management block. The STS management block routes the traffic to the ATM cell management block and to the non-ATM payload management block, according to the traffic type. The STS management block also provides the UPSR protection for the STS-1s and ULSR protection for the STS-Mc. The ATM cell management block maps the add ATM cells received from the ATM ports into the STS-Mc signal, and delineates the cells from the STS-Mc to present them to the ATM ports. The non-ATM payload management block routes STM VTs or STS-1s to/from the non-ATM ports.

This is a continuation of application Ser. No. 08/783,869, filed Jan.16, 1997 now U.S. Pat. No. 6,256,292 which is incorporated herein byreference claims benefit of 60/021,575 filed Jul. 11, 1996.

FIELD OF THE INVENTION

The invention is directed to a line switched ring for carryingasynchronous transfer mode (ATM) traffic or mixed ATM and synchronoustransfer mode (STM) traffic for standardized communication of multimediainformation, and more particularly to a self-healing line switched ringusing a unidirectional path for ATM traffic.

BACKGROUND OF THE INVENTION

The synchronous optical network (SONET) is a standard for a synchronoustelecommunication signal used for optical transmission, based on thesynchronous digital hierarchy (SDH). SONET is a physical carriertechnology, which can provide a transport service for ATM, SMDS, framerelay, T1, E1, etc. As well, SONET provides the ability to combine andconsolidate traffic from different locations through one facility(grooming), and reduces the amount of back-to-back multiplexing. Moreimportantly, network providers can reduce the operation cost of theirtransmission network by using the notably improved operation,administration, maintenance and provisioning (OAM&P) features of SONET.

The SONET standards ANSI T1.105 and Bellcore GR-253-CORE, define thephysical interface, optical line rates known as optical carrier (OC)signals, a frame format, and an OAM&P protocol. The user signals areconverted into a standard electrical format called the synchronoustransport signal (STS), which is the equivalent of the optical signal.The STS-1 frame consists of 90 columns by 9 rows of bytes, the framelength is 125 microseconds. As such, an STS-1 has a bit rate of 51.840Mb/s. Higher rates (STS-N, STS-Nc) are built from this one, and lowerrates are subsets of this. The lower rate components, known as virtualtributaries (VT), allow SONET to transport rates below DS3.

Requests and acknowledgements for protection actions are transmitted inthe APS bytes in the SONET overhead, i.e. K1 and K2 bytes. The K1 bytecommunicates a request for a switch action. The first four bits of K1indicate the switch request priority and the last four indicate thedestination ring node identification (ID). The K2 byte indicates anacknowledgement of the requested protection switch action. The firstfour bits of K2 indicate the source ring node ID and the last four bitsindicate the action taken by that node.

A SONET add/drop multiplexer multiplexes various STS formatted inputstreams onto optical fiber channels. The STS signals are carried by anoptical carrier, which is defined according to the STS that it carries.Thus, an STS-192 signal is carried by an OC-192 optical signal.

The topology of the SONET network can be a linear point-to-pointconfiguration, or a ring configuration.

A linear topology can only protect against single fiber link failures. A“1:1” linear system has an equal number of working and protection links;a “1:N” linear system has N working channels and one shared protectionchannel.

Lately, rings have become the topology of choice in fiber deployment.The prime motivator for rings versus linear transport is highersurvivability. A ring protects against simultaneous failure of theprotection and working fibers (i.e. cable cuts) and saves intra-ring andinter-ring pass-through traffic during node failure/isolation. Ringsoffer cost effective transport while delivering enhanced networksurvivability.

Currently, two type's of rings are used, namely, unidirectional pathswitched rings (UPSR), and bidirectional line switched rings (BLSR). TheUPSRs are currently used in access networks and therefore they are builtfor lower rates, such as OC-3, which are sufficient for access linkdemands. UPSR protection switching is done at the SONET path level. Theoperation of UPSRs is standardized by the BellCore GR-1400-COREstandard, and there are OC-3/12 rate products available.

The BLSR are currently used in the backbone networks and therefore theyare built for higher rates such as OC-48. Switching is done at the SONETline layer. The operation of BLSRs is standardized by the BellCoreGR-1230-CORE standard, and there are OC-12/48 rate products available.

The asynchronous transfer mode (ATM) forms the basis for switching inbroadband networks. ATM convergence functions permit switching of voice,video and data traffic through the same switching fabric. It multiplexesuser information into fixed lengths cells of 53 bytes, with 5 bytesforming a header.

With the constant growing needs for enhanced services in informationtransmission networks, more efficient transport for bursty trafficcarried in ATM cells is needed. There is also a need to simplify andstandardize the access link while also obtaining protection of theaccess traffic. Current practice is to dedicate an entire facility tothese new services, such as one STS-1 per customer, where the entirepayload is cell based.

While synchronous transfer mode (SIN) access traffic can beadvantageously carried on a UPSR, there are disadvantages for ATMtraffic. To carry ATM traffic and have the transport vehicle benefitfrom, and not restrict the bandwidth on demand feature that ATM canprovide, the optimum approach is to share a large block of bandwidthbetween nodes around the ring. In this way, a virtual path (VP) added ata node uses the bandwidth it needs within the large block, rather thanusing, say, a virtual tributary (VT) where its burst rate will besignificantly limited. The shared bandwidth block could be an STS-Nc,where N=1, 3, 6, or higher rates.

This assumes a UPSR with an STS-Nc granularity selector at thetail-ends. With an STS-Nc passing around the UPSR from node to node, thering bandwidth is quickly exhausted, as each node must source the STS-Ncin different timeslots in order to leave protection timeslots availablefor other nodes. The UPSR could, in theory, reuse the same workingtimeslots for the STS-Nc between nodes by operating unprotected andleaving protection to the ATM layer. However, no standardized scheme yetexists at the ATM layer which can provide the 60 ms protection speedstypical of SONET. It is apparent that the two goals of bandwidthefficiency and SONET protection are mutually exclusive in the context ofATM traffic on a UPSR.

A BLSR can carry an STS-Nc node to node in a bandwidth efficient manner.As well, the BLSR can protect any service type since it switches at theline layer. However, the BLSR is not an economical solution for ATMaccess. This is because for an OC-3 line rate ring, a two-fiber BLSR(2F-BLSR) is not realizable and four-fiber BLSRs (4F-BLSR) are currentlynot available at the OC-3 rate. An OC-3 4F-BLSR, if it becomesavailable, would carry twice the bandwidth of an OC-3 UPSR, but almostdoubles the cost of the fiber and equipment required, and wouldtherefore be uneconomical for the majority of access applications.Similarly, an OC-12 UPSR or 2F-BLSR, which is typically the next step inupgrading an access applications, carries four times the bandwidth of anOC-3 UPSR, but again, at greater than double the OC-3 cost.

Due to the working timeslot reuse capability, a BLSR always provides theoptimum use of bandwidth for a given traffic pattern. However, a complexautomatic protection switching (APS) protocol is necessary, whichresults in longer switching times than for a UPSR. In addition,protection is not optional on a per-path basis. It is also to be notedthat for a bidirectional homing traffic pattern typical of the accessnetwork, a UPSR is as bandwidth efficient as a BLSR. For a mesh trafficpattern typical of the backbone network, a BLSR is more bandwidthefficient.

In addition, regardless of the line rate or 2F/4F type, a BLSR mustperform the ATM add/drop functionality for two bidirectional channels(e.g. east and west). ATM chip sets available today are designed forterminals, or a single bidirectional channel (e.g. east or west). It isexpected that the evolution to ATM chips consolidating add/dropfunctionality for two bidirectional channels will be eventuallyavailable, but again at higher costs.

In conclusion, there is no standardized survivable access vehiclecurrently available which can efficiently carry ATM or mixed ATM/STMtraffic.

SUMMARY OF THE INVENTION

The invention provides a novel type of ring for ATM and mixedATM/non-ATM traffic, employing the UPSR protocol for non-ATM traffic anda new protocol for ATM traffic.

For non-ATM access traffic, the UPSR provides a simple protectionprotocol, is bandwidth efficient and works well with path based planningtools used in access. Protection can be optional on a per-path basis. Inaddition, the UPSR has a fast protection time.

For ATM access traffic, none of the existing rings, UPSR or BLSR,satisfies the goals of bandwidth efficiency, SONET protection andeconomy. The UPSR, predominant in the access network, could carry ATMtraffic and be bandwidth efficient but without protection by SONET. IfSONET protection is implemented for ATM traffic on the UPSR, bandwidthutilization becomes inefficient. A BLSR could carry ATM traffic, bebandwidth efficient and provide SONET protection. However, the cost ofproviding such a product is high for an access application. Also, thereare no BLSR products possible for 2F or available for 4F at the OC-3rate, which is currently the predominate rate in the access network.

According to this invention, ATM access traffic is carried over aunidirectional path using a line switched ring. The unidirectional lineswitched ring (ULSR) of this invention can be implemented at any linerate, including for example an OC-3 line rate, and at costs lower thanfor BLSR rings. The ULSR provides a unidirectional traffic flow for bothATM and non-ATM traffic, which allows for traffic management andoperation commonality. A ULSR is as bandwidth efficient as a BLSR orUPSR for homing traffic. Further, a VP in a ULSR could burst up to theSTAN rate, rather than the STS-N/2 limitation with a 2F BLSR.

The ULSR is not currently defined by standards and no SONET products ofthis type are available.

According to a particular aspect of his invention, a BLSR at a high linerate, such as for example OC-12/48/192, may be used to carry symmetricalATM traffic in one direction around the ring and, for example, a TVbroadcast in the other direction.

It is an object of the present invention to provide a self-healing lineswitched fiber optic ring to efficiently carry ATM or mixed non-ATM andATM traffic over a unidirectional path, using SONET technology. The ringis primarily for use in an access network, but it can also be used in abackbone network.

Another object of the invention is to provide a switched ring forcarrying ATM traffic with STS-Nc granularity, and non-ATM traffic withVT and/or STS-1 granularity. Thus, the ring of the invention provides asingle vehicle for carrying both non-ATM and ATM traffic at all nodes onthe ring. Since the ring is unidirectional, like a UPSR, management ofthe non-ATM traffic would be necessary.

Another object of this invention is to provide a method for using a BLSRor a unidirectional line switched ring (ULSR) for bidirectional trafficvia unidirectional transport around the ring. This structure preservesthe bidirectional nature of the service at the interfaces, however, itoffers cost advantages in the design of the transport layer ring ADMs.The control of the cell add/drop function is implemented such that thecell transport function operates as a physically distributed switch. Foran access application with homing traffic, the ring provides thegrooming and consolidation of cells between the customer access and theATM edge switch.

Accordingly, the invention comprises a method for communicatinginformation over a SONET line switched ring having a plurality (K) ofcommunication terminals connected to a first and a second transmissionline, comprising the steps of at each terminal (k), inserting anoutgoing signal onto the first transmission line along a first directionof transmission defined from the terminal (k), towards an adjacentterminal (k+1), wherein (k) is an integer between 1 and (K) giving thesequential position of the node (k) in the ring; at each the terminal(k), receiving an incoming signal over the first transmission line froman adjacent terminal (k−1), along the first direction of transmission;and operating the plurality of communication terminals according to aunidirectional protection protocol upon detection of a failure conditionin the incoming signal.

The invention further comprises a method for transporting a SONETformatted asynchronous transfer mode (ATM) signal on a unidirectionalline switched ring comprising the steps of connecting a plurality (K) ofnodes in a ring network provided with a working transmission lineassociated with the first direction of transmission and a protectiontransmission line associated with a second direction of transmission,opposite to the first direction; detecting at a node (k) an error signalreceived from a node (k−1) located adjacent to the node (k) and upstreamwith respect to the first direction; at the node (k), generating astatus change request upon receipt of the error signal, and transmittingthe status change request on the working and protection transmissionline; and restructuring all the nodes of the ring to operate accordingto one of a working transmission line failure (WTLF), a node failure(NF), a protection transmission line failure (PTLF), and a working andprotection transmission line failure (WPTLF) configuration, upon receiptof the status change request.

The invention further comprises a method for communicating informationon a bidirectional line switched ring (BLSR) configuration including aplurality (K) of ring nodes connected by a first and a secondtransmission line, comprising the steps of deploying the BLSR in ahoming-type configuration, each node (k) having an incoming pathassociated with a node drop direction and an outgoing path associatedwith a node add direction: at a first node (q) where q≠k of the BLSR,transmitting a first SONET formatted signal along the first transmissionline, the first SONET formatted signal having a bandwidth K×BW; at eachnode (k) of the BLSR, receiving the first SONET formatted signal fromthe first transmission line, extracting same over a respective incomingpath, and re-transmitting the first SONET formatted signal over arespective outgoing path back into the first transmission line; at eachthe node (k), inserting a respective outgoing SONET formatted signalinto the second transmission line over the respective outgoing path,each outgoing SONET formatted signal comprising traffic formatted at arespective node (k), and having a bandwidth BW; and at each the node(k), extracting a respective incoming SONET formatted signal receivedfrom the second transmission line over the respective incoming path,each incoming SONET formatted signal comprising traffic addressed to therespective node (k), and having the bandwidth BW.

The invention further comprises a node for a SONET line switched ringcomprising a first ring interface with a first working port forreceiving an incoming optical signal OC-Mc from a working fiberassociated with a first direction of transmission, and converting sameinto an incoming STS-Mc; a second ring interface with a second workingport for converting an outgoing STS-Mc into an outgoing optical signalOC-Mc and transmitting same over the working fiber; an ATM cellmanagement block for routing an output ATM cell extracted from theincoming STS-Mc as one of a drop ATM cell and a passthrough ATM cell,and multiplexing the passthrough ATM cell and an add ATM cell into theoutgoing STS-Mc; and an STS management block for routing the incomingSTS-Mc between the first ring interface and the ATM cell managementblock, and the outgoing SSMC between the ATM cell management block andthe second ring interface.

The invention further comprises a node for a SONET line switched ringcomprising a first ring interface with a first working port forreceiving a SONET formatted incoming optical signal from a working fiberassociated with a first direction of transmission, and converting sameinto an incoming non-ATM signal and an incoming ATM signal; a secondring interface with a second working port for converting an outgoingnon-ATM signal and an outgoing ATM signal into an outgoing SONETformatted optical signal and transmitting same over the working fiber;an ATM cell management block for processing and transmitting an outputATM cell extracted from the incoming ATM signal as one of a drop ATMcell and a passthrough ATM cell, and multiplexing the passthrough ATMcell and an add ATM cell into the outgoing ATM signal; an STS managementblock for routing the incoming non-ATM signals as one of an outputnon-ATM signal and a passthrough non-ATM signal, routing the passthroughnon-ATM signal and an input non-ATM signal as the outgoing non-ATMsignal and routing the incoming and outgoing ATM signals between thefirst and the second ring interfaces and the ATM cell management block;and a non-ATM payload management for processing and transmitting theoutput non-ATM signal to a non-ATM port, and for processing andtransmitting the outgoing non-ATM signal to the STS management block.

Advantageously, the ring according to this invention equals thebandwidth efficiency of a BLSR for homing traffic and provides SONETprotection while overcoming cost concerns. A VP could burst up to a ringline rate N, whereas in a 2F-BLSR the burst rate would be limited to aSTS-N/2 rate. In some cases, the unidirectional add/drop feature of thering of the invention may offer twice the bandwidth for access rings,while offering protection for the traffic in the standard manner. Thus,if for example, a standard BLSR can carry a STM STS-6 in one directionand two ATM STS-3c in the other direction, the ring of the presentinvention can carry in addition two STS-3s of video, which travel in adrop-and-continue fashion.

Since the ring provides the same unidirectional traffic flow for bothATM and non-ATM traffic, traffic management commonality may be obtained.

Still another advantage of this new type of ring is that minimum changesto the BellCore GR-1230-CORE standard would be required to accommodatethe ULSR APS protocol for protection of ATM traffic.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features and advantages of theinvention will be apparent from the following more particulardescription of the preferred embodiments, as illustrated in the appendeddrawings, where:

FIG. 1 illustrates the applicable OSI layers for ATM and SONET;

FIG. 2A shows a simple ATM configuration;

FIG. 2B shows a simple SONET configuration;

FIG. 3A is a block diagram of a unidirectional path switched ring (UPSR)in normal operation;

FIG. 3B is a block diagram of a UPSR showing protection actions during acable cut;

FIG. 4A is a block diagram of a 2F-BLSR in normal operation;

FIG. 4B is a block diagram of a 2F-BLSR showing protection actionsduring a cable cut;

FIG. 5A is a block diagram of a unidirectional line switched ring (ULSR)of this invention, in normal operation;

FIG. 5B is a block diagram of a ULSR showing protection actions during acable cut;

FIG. 6A shows a ULSR node in the idle state;

FIG. 6B shows a ULSR node in the passthrough and full passthrough state;

FIG. 6C shows a ULSR node in the bridged state;

FIG. 6D shows a ULSR node in the switched state;

FIG. 7 is a flow-chart for operation of the ULSR ring when the workingor/and protection fiber(s) is/are interrupted;

FIG. 8 illustrates an example of a STS-3c signal formed with mixed ATMand STM traffic;

FIG. 9 illustrates a block diagram of a node according to thisinvention;

FIG. 10 is a block diagram of the ATM cell management block of the nodeillustrated in FIG. 9;

FIG. 11 is a flow-chart illustrating the squelching scheme for theswitched ring of FIGS. 5A and 5B;

FIG. 12 shows an example of the ring of the invention connected in anaccess level transmission network;

FIG. 13A shows the transport of ATM traffic according to this inventionon a prior art BLSR; and

FIG. 13B shows the transport of ATM traffic on a unidirectional channelusing the BLSR of FIG. 13A.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIGS. 1, 2, 3A, 3B, 4A and 4B are provided for defining and illustratingsome of the terms necessary for describing the present invention and itsmode of operation.

FIG. 1 shows the relationship between the ATM layer and the SONETphysical layer. The physical layer for ATM can be made up of a SONETcarrier and the ATM operations are transparent to SONET.

FIG. 2A shows a basic linear ATM configuration, illustrating a typicalpoint-to-point ATM topology, where the connections are identifiedthrough virtual channel identifiers (VCI) and virtual path identifiers(VPI) in the ATM cell header. A virtual channel connection (VCC) 200 hasend-to-end significance between end users A and B. Switching in the ATMnetwork is illustrated at 211, 212, and 213. A virtual path connection(VPC) has significance between adjacent ATM devices, as illustrated at221, 222, 223, and 224. From an input port, a cell with a given VPI/VCIis mapped to an output port and assigned a potentially differentVPI/VCI.

Switching is performed very quickly through the use of a routing table.ATM switches are available today with a total switching capacity in theorder of Gb/s. The virtual channels (VC) and virtual paths (VP) of ATMrun on the SONET physical layer, as shown in FIG. 1.

FIG. 2B shows a basic linear SONET configuration illustrating some SONETterms. The physical layer of SONET is modelled on three major entities:transmission path, multiplex section and regenerator section, each layerrequiring the services of all lower layers to perform its own function.These layers correspond to SONET path, line and section layers shown inFIG. 1.

The section layer deals with the transport of multiplexed signals acrossa physical medium. A section is a portion of the transmission facilitybetween two section terminating equipments (STE), such as regeneratorsand terminals. Functions include adding the section level overhead(SOH), framing, scrambling, section error monitoring and an embeddedcommunication channel. FIG. 2B shows three sections, each definedbetween two successive STEs, namely, section 301 between terminal 320and regenerator 330, section 302 between regenerator 330 and add-dropmultiplexer 350, and section 303 between add-drop multiplexer 350 andterminal 360.

The line layer provides synchronization and multiplexing for the pathlayer. A line is a portion of the transmission facility between twoconsecutive line terminating equipments (LTE). The LTEs could beadd-drop multiplexers or terminals (TM). An ADM canmultiplex/demultiplex various inputs from an optical signal. It accessessignals that need to be dropped or inserted at that site, the rest ofthe traffic continuing straight through. FIG. 2B illustrates line 304between terminal 320 and ADM 350, and line 305 between ADM 350 andterminal 360.

The path layer deals with the transport of services, such as DS1 or DS3,between path terminating equipments (PTE). The PTE could be ADMs orterminals serving routers, bridges, PBXs or switches. The main functionof the path layer is to map the services and path overhead (POH) intoSTS-1s, which is the format required by the line layer. FIG. 2B shows apath 306 of a DS3 mapped STS-1 which originates at terminal 320 and isdelivered at terminal 360. In another example, the circuit betweenterminals 320 and 350 transports an OC-12 signal, with DS-ls being addedat nodes 370 and 380 to form the OC-12, in which case path 307 does notstart or begin at nodes 320 and 350, but at nodes 370 and 380. The DS-1sare transported in VT1.5s from terminal 370 to terminal 380 on path 308.

A fourth layer, not illustrated, is the physical medium layer. It is thebasic physical layer that provides optical transmission at a given bitrate. Issues dealt with at this layer include optical pulse shape, powerlevels, and wavelength. Electro-optical units communicate at this level.

In this specification, the term “normal operation-conditions” definesthe operation of the ring of the present invention when the trafficbetween the ring nodes is directed along the working fiber and theprotection fiber is idle, or used for lower priority extra-traffic (ET).The term “failure operation conditions” defines the operation of thering when the connection between some ring nodes is interrupted due to acable cut or a node failure. The terms “unidirectional” and“bidirectional” protection switching refer to modes of the protectionprotocol and should not be confused with the terms “unidirectional” and“bidirectional” connections. The term “transmission line” refers to thephysical medium that connects two terminals, which is fiber optics inthe following examples.

The term “line switched ring configuration” defines a ring where theprotection switching is done at the SONET line level, while the term“path switched ring configuration” defines a ring where the protectionswitching is done at the SONET path level. These terms are explainednext in further detail in connection with FIGS. 3A, 3B, 4A and 4B.

FIGS. 3A and 3B illustrate an example of a UPSR with six nodes (PTEs)and FIGS. 4A and 4B shows a BLSR with six nodes (LTEs).

The UPSR shown in FIG. 3A interconnects ADMs A, B, C, D, E, and F alongworking fiber (W), in a clockwise direction, and ADMs A, F, E, D, C, andB along protection fiber (P) in a counterclockwise direction. Theworking fiber (W) between ADMs A and D has been denoted with numeral 1,while the working fiber between D and A has been denoted with 2. Theprotection fiber (P) between A and D is denoted with 4 and theprotection fiber between D and A is denoted with 3.

The AD signal normally travels on working fiber 1 in the directionillustrated by arrow 5. The DA signal normally travels on working fiber2 in the direction illustrated by arrow 5′. The ring provides apermanent head-end bridge at path layer for VTs and STSs, as illustratedat ADMs A and D by numerals 6 and 7. The switches at the tail-ends 8 and9 are connected to the working fiber 2 and 1 respectively, such that thetraffic AD from ADM A exits at ADM D and the traffic DA from ADM D exitsat ADM A. Note that this is a bidirectional-connection, yet the trafficflow is unidirectional.

If fibers 2 and 4 are cut as shown in FIG. 3B, the traffic on workingfiber 2 is interrupted. Switch 8 at ADM A switches to the protectionfiber once ADM A determines that the signal on fiber 3 is of a betterquality than the signal on working fiber 2. Now the traffic DA flows onprotection fiber 3, as shown by arrow 5′, and exits at ADM A throughswitch 8. Switch 9 at ADM D remains unchanged, and the traffic from ADMA to ADM D continues to flow along working fiber 1.

With this type of ring, the signal is always present on both working andprotection fibers and no tail-to-head signaling is necessary; onlytail-end switching is required. This results in fast switching times. Aprimary disadvantage of this type of ring, however, is that theprotection fiber cannot be used to carry extra-traffic (ET).

FIG. 4A shows a two-fiber (2F) BLSR which is currently used in thebackbone transport link. The ring nodes comprise line terminatingequipment (LTE), and the switching is done at the line level. Since thering is bidirectional, both fibers between the nodes are used for theworking traffic. For a 2F-BLSR, the same fibers must also haveprotection capacity allocated within them.

Under normal conditions, shown in FIG. 4A, bidirectional traffic betweenADMs A and D takes place in the working time-slots on fibers 2 and 4.Fibers 1 and 3 are also available for additional traffic between ADMs Aand D. Node A and node D are connected to fiber 4 for transport ofsignals from A to D, and to fiber 2 for transport of signals from ADMs Dto A, as shown by arrow 5.

FIG. 4B illustrates the case of a cable cut on fibers 2 and 4 betweennodes E and D. In this case, the traffic AD/DA affected by the cable cutis redirected as described next. ADM D/E detects the line failure andbegins protection signalling with ADM E/D on fibers 2-1 and 3-4. ADMs Dand E are called the switching nodes.

The adding of traffic at A remains unchanged. The traffic from ADM A forD arrives at E in working time-slots on fiber 4. When node E detects thecable cut, it returns the traffic for node D towards node A in theprotection time-slots on fiber 2 to be dropped at D from fiber 1. Tothis end, drop selector 9 at ADM D selects protection timeslots in fiber1.

When node D detects the cable cut, the traffic added at D for A isredirected in the protection timeslots of fibers 34, until reaching nodeE, where it is looped back towards A in working time-slots on fiber 2.The dropping of traffic at A remains unchanged.

The passthrough traffic from ADM C towards ADM F travelling normally inthe working timeslots on fibers 1 and 2, for example, is looped back atADM D in fiber 3 protection timeslots. To this end, the switching node Dconnects working passthrough timeslots in fiber 1 to protectiontimeslots in fiber 3. Then, the looped back traffic arrives at switchingnode E in protection timeslots on fiber 4 where it is looped such thatit is delivered to ADM F in working timeslots of fiber 2.

Thus, this type of ring provides for on-demand head-end bridge at theline layer, the tail-end selection depends on the head-end bridge, andthere is tail-head signaling. The switching action only occurs at thenodes on either side of the fault, D and E in the example of FIG. 4B.

Since for a BLSR configuration the protection timeslots are only usedduring a protection switch, they can be used for lower priority ET.

FIG. 5A illustrates a ULSR according to this invention with six nodes A,B, C, D, E and F. The example shows traffic connectivity between nodes Aand D under normal working conditions. Traffic added at node A anddestined for node D flows clockwise (arrow 5) on the working fiber 1.The return path from node D to node A is in the same direction (arrow5′) on working fiber 2. The flow of traffic on the second fiber, theprotection fiber, is in the opposite direction.

All nodes are in an idle state, as further illustrated in FIG. 6A fornode A In this state, protection traffic is not established; fibers 3and 4 are idle. Add traffic is inserted onto working fiber span 20 atnode A, and the passthrough traffic is transferred from working fiber 2onto working fiber 20. The drop traffic is selected at node A by dropselector 8 from working fiber 2.

It is apparent that the unidirectional traffic flow is similar to theworking traffic flow in the UPSR illustrated in FIGS. 3A and 3B, exceptthat there is no permanent protection bridge 6 or 7 at any node. Trafficis always added in the same direction and on the same fiber at the nodesillustrated FIGS. 5A and 5B.

On the other hand, the node states of the ULSR of FIGS. 5A and 5B aresimilar to those of the BLSR of FIGS. 4A and 4B and the protectionactions are similar, except that they are unidirectional. Thus, there istail-to-head signaling to request a bridge, which also triggers theintermediate nodes to drop into passthrough state. In general, the tailmust see the head-end bridge prior to switching. The switching nodesinvolved in bridging/switching are on either side of the fault, asdisclosed next with reference to FIGS. 5B, 6A-6D and 7. At each node,the incoming signals on both the working and the protection fibers aremonitored for detecting signal failure (SF) or signal degradation (SD).If fiber span 10 between nodes C and D is cut, as shown in FIG. 5B, nodeD will detect the absence of the respective incoming (W) signal. In theexample of FIG. 7, nodes D and C monitor the respective incoming signalfor detecting a signal failure in step S1. As indicated above, in caseof a cable cut between nodes C and D, node D would initiate the “SF onW” branch in step S2, while node C would initiate the “SF on P” branchin step S10. Node D also establishes in steps S2 and S10 which fiber wasaffected, i.e. the working fiber 10, and/or the protection fiber 4. Ifworking fiber 10 has been cut, node D requests in step S3 a bridge atnode C by sending a long path request (LPR) on fiber 2 and a short pathrequest (SPR) on fiber 3 around the ring.

When node C receives the long path request, it assumes a bridged statein step S4. FIG. 6C shows node C in the bridged state, where the nodebridges its add traffic, normally sent towards node D on working fiber10, onto the protection fiber 3 towards the destination node(s).

Node C also loops the passthrough traffic from working fiber 1 toprotection fiber 3. Traffic destined to any of nodes B, A, F, or E, islooped at node D from protection fiber 4 and arrives at the destinationnode(s) on working fiber 2, 20. The node C drop traffic is not affected.

In step S5, intermediate nodes B, A, F and E assume a full-passthroughstate when they see the LPR. FIG. 6B shows node B, for example, in afull passthrough state (solid lines), which is the mode of anintermediate node during a ring switch. In the full passthrough state,connection is established along the protection fiber 3 for accommodatingprotection traffic No extra traffic (ET) is possible in this state. Nochange occurs to working traffic flow, fibers 20 and 1.

If, in step S6, node D receives an APS indication that node C is in thebridged state, then node D assumes a switched state per step S7. FIG. 6Dshows node D in the switched state. The traffic normally dropped fromworking fiber 10 is selected from the protection fiber 4. Thepassthrough traffic normally received from node C on working fiber 10 isalso selected from the protection fiber 4 and directed on working fiber2 towards the destination node(s) E, F, A, B. The add traffic isunaffected.

If the failure is cleared, as determined in step S8, all nodes return tothe idle state in step S9.

The signaling between the ring nodes for the case where the error is ofan SD type is similar to that above for an SF type.

If node C determines in step S10 that the interruption affects only theprotection fiber 4, node C requests a K-byte passthrough state in stepS11, and the intermediate nodes assume the K-byte passthrough state instep S12. This is a subset of the full passthrough state; thedifferences are illustrated in FIG. 6B by the dotted line. In thisstate, the K-bytes received on both working and protection fibers arepropagated through the node. Although the K-bytes on the protectionchannel are passed through, extra traffic access is still available,which is different from the full pass-through state.

FIGS. 6A to 6D illustrate the various states of a ULSR node, which weredescribed in connection with FIGS. 5A and 5B. FIG. 6A shows ring node Ain the idle state, FIG. 6B shows ring node B in the passthrough or fullpassthrough state, FIG. 6C shows ring node C in the bridged state, andFIG. 6D shows ring node D in the switched state.

Since the ring of the invention has the similar bridging, passthrough,and switching functionality of a BLSR, a protection protocol similar tothe GR-1230-CORE standard for a BLSR may be used, but without the rulesassociated with bidirectional actions and span switching.

The SONET APS bytes K1 and K2 are located in the line overhead (LOH) ofthe SONET frame, and are used for APS signaling between line levelentities. These bytes are defined only for the first STS-1 of an STS-Nsignal.

The GR-1230-CORE BLSR K1-2 values can be modified for ULSR usage, asshown next. Tables 1 and 2 show the K-Bytes for these two types ofrings, where the altered values are highlighted.

TABLE 1 K1 Assignment Indication (BLSR) Indication K1 bits Value GR-1230(ULSR) 1-4 1111 LP-S, SF-P LK-P 1110 FS-S SF-P 1101 FS-R FS 1100 SF-S —1011 SF-R SF-W 1010 SD-P SD-P 1001 SD-S — 1000 SD-R SD-W 0111 MS-S —0110 MS-R MS 0101 WTR 0100 EX-S — 0011 EX-R EX 0010 RR-S — 0001 RR-R —0000 NR NR 5-8 DESTINATION NODE ID

As seen in Table 1, all the span codes (LP-S, FS-S, SF-S, SD-S, MS-S,EX-S) are eliminated. Lockout of protection is assigned highest priorityfollowed by SF on protection; this is similar to linear APS. The reverserequest codes (RR-S, RR-R) are eliminated.

TABLE 2 K2 Assignment Indication (BLSR) Indication K2 bits Value GR-1230(ULSR) 1-4 Source Node ID 5 1 Long Path Long Path 0 Short Path ShortPath 6-8 111 L-AIS L-AIS 110 L-RDI L-RDI 101 — — 100 — — 011 ET ET 010Br & Sw Switched 001 Bridged Bridged 000 Idle Idle

It is seen in Table 2 that the Bridged & Switched code collapses to theSwitched code.

FIG. 8 illustrates an example of an STS-3 signal formed with mixed ATMand non-ATM traffic. In this example, STS-1 #1 is an ATM mapped STS-1which may carry up to 65,536 VCs. The ATM mapped STS-1 is protectionswitched per the ULSR scheme described above.

The second and third STS-Is of the FIG. 8 example carry non-ATM trafficwith VT and STS-1 mapping, respectively. The UPSR protection protocol isused for the non-ATM traffic.

In another example, an OC-12 structure may be obtained with some STS-1scarrying STM traffic of VT/STS-1 mapping and with some ATM traffic inincrements of STS-Is or STS-3cs.

In general, the signal received/sent by a node from/to the ring isdenoted in the following with STS-N. The non-ATM SONET-formatted signalis denoted with STS-W, comprising W×STS-1s. The ATM SONET formattedsignal is denoted with STS-Mc, where M represents the number of STS-1sconcatenated for transporting a cell-based payload. An STS-Mc accordingto this invention also represents ATM traffic formatted as m×STS-1s. Itis to be understood that W+M=N, which is the bandwidth allocated to therespective node, and N, W and M are integers taking values 1, 2, 3, etcFIG. 9 illustrates a block diagram of node A of FIGS. 5A and 5Baccording to this invention.

The term “incoming” is used in the following to designate the trafficdirection from the ring to node A, while the term “outgoing” is used todesignate the traffic direction from the node to the ring. The term“add” designates the direction from the trib ports to the node, while“drop” defines the direction from the node to the trib ports.

O/E interface 50 comprises an input working port and an outputprotection port. The input working port converts an incoming SONETformatted optical signal OC-N, received on working fiber 2, to anincoming electrical signal on line 51. The output protection portconverts an outgoing electrical signal on line 52 to an outgoing SONETformatted optical signal OC-N on protection fiber 4. O/E interface 60comprises an output working port, and an input protection port. Theoutput working port converts the outgoing electrical signal on line 52to an outgoing optical signal on working fiber 20, while the inputprotection port at interface 60 converts an incoming optical signalreceived on fiber 3 to an incoming electrical signal on line 51.

Interfaces 50 and 60 perform SONET physical layer operations. Withrespect to the incoming working traffic, the input working ports atinterfaces 50 and 60 are also responsible for descrambling the incomingsignal, stripping the section overhead (SOH) and the line overhead(LOH), and transmitting the incoming non-ATM STS-1s and the ATM STS-Mcto an STS management block 70. The incoming APS bytes are detected andvalidated, and the K-bytes are provided to an STS protection controllerblock 115. The input working port also performs clock recovery andsynchronization of the STS paths.

Since working port at the interface 50 accesses the LOH, it isresponsible for detecting errors in the incoming signal and performingcomparisons with a provisioned signal degrade threshold. Exceeding thethreshold constitutes an SD in protection terminology. Also, eachinterface 50 and 60 is responsible for detecting when its partner porthas failed due to an equipment failure, removal, power loss, or localmicrocontroller failure.

With respect to the outgoing working traffic, the output working port atinterface 60 is responsible for receiving the outgoing STS-1s and STS-Mcfrom the STS management block 70, generating the LOH and SOH, scramblingand converting the electrical signal to an outgoing optical signal OC-N,and transmitting it on fiber 20. The output working port also performsclock synthesis, synchronization, and updates the outgoing APS bytes.

The input and output protection ports at interfaces 50 and 60 performthe same functions as above for the termination and generation of theprotection channel.

STS management block 70 supervises the communication of the incoming andoutgoing signals between node A and the ring, according to the trafficmode.

For the incoming direction, STS management block 70 receives theincoming signal from interface 50 or 60 and routes the output ATM mappedSTS-Mc to an ATM cell management block 90. As well, block 70 delineatesthe non-ATM STS-1s from the STS-W and routes them to a non-ATM payloadmanagement block 80. STS management block 70 also passes through thetraffic that is not addressed to node A. For the outgoing traffic, STSmanagement block 70 receives and routes an input ATM mapped STS-Mc fromthe ATM cell management block 90, and/or an input VT/STS-1 structuredpayload from the non-ATM payload management block 80.

Block 70 is responsible for UPSR protection for the non-ATM STS-1s, andthe ULSR protection for the STS-Mc, under control of a STS protectioncontroller 115. Since protection switching at node A is unidirectional,block 70 routes the outgoing signal always on output line 54, and theincoming traffic is received always from line 53. Prior art BLSRs need asecond input and output line, since the traffic in this case is insertedor received to/from both directions.

For the input direction, ATM cell management unit 90 multiplexes theinput cells received from the ports 92, 93, generates the path overhead(POH) and assemblies an ATM structured input STS Mc, which is presentedto the STS management 70 on line 71. For the output direction, block 90delineates the output cells from the incoming STS-Mc, terminates thePOH, and routes the drop/passthrough cells, as will be described infurther detail in connection with FIG. 10. The ATM cell management block90 is interconnected with ATM ports 92, 93 in a star configuration.

ATM cell management block 90 also performs the protection for the ATMports.

Non-ATM payload management block 80 may be for example a VT managementblock in the case of a VT/STS-1 structured payload. For the inputdirection, block 80 receives from bus 81 the add non-ATM traffic, suchas VTs/STS-1s, generates the POH, maps the traffic into an input STS-1,and provides the non-ATM structured input payload to STS managementblock 70 on line 84, for routing to the ring. For the output direction,block 80 receives the non-ATM output ST-1s from STS management block 70on line 85, strips the POH, and routes the drop traffic to the bus 81.

Block 80 provides the UPSR protection for VTs under the control of anon-ATM protection controller 116 and the protection for the non-ATMDS1s, and/or DS3s added/dropped at ports 82 and 83.

STS protection controller 115 bases its switching actions on the K-bytespassed from interfaces 50 and 60. Protection controller 115 manipulatesthe incoming and outgoing K-bytes and supervises the protectionswitching. For the non-ATM traffic, such as non-ATM STS-1s, it uses theUPSR protocol, and for the ATM traffic, such as the ATM STS-Mc, it usesthe ULSR protocol. User protection commands, user provisioning anddefaults are accepted by the STS protection controller. It also receivesequipment status reports from blocks 50 and 60.

Setting the node into a bridged, switched or passthrough state accordingto the information in the K1 and K2 bytes of the incoming STS-Mc for ATMtraffic takes place as disclosed in connection with FIGS. 5A and 5B.Thus, when node A assumes a switched state, the passthrough ATM trafficreceived from the protection fiber 3 are output onto the working fiber20. The drop traffic is selected from the protection fiber 3 and the addtraffic remains unaffected. In a bridged state, the passthrough ATMtraffic is routed from the working fiber 2 onto protection fiber 4, theadd traffic is bridged on protection fiber 4, and the drop traffic isselected from working fiber 2. In the full passthrough state, thepassthrough cell mapped STS-Mcs are transported through the node fromthe protection fiber 3 to fiber 4, add traffic is still inserted onworking fiber 20 and traffic is dropped from working fiber 2.

STS protection controller block 115 also performs protection switchingfor the STM STS-1s, according to the UPSR switching protocol illustratedin FIGS. 3A and 3B for the UPSR rings. STS protection controller block115 also monitors the ATM trib ports 92, 93, to control ATM tribequipment protection.

Non-ATM protection controller unit 116 controls the UPSR bridge, switch,and passthrough functions for the non-ATM traffic (for example STM VTs)and tributary equipment protection for the ports 82, 83, which could beDS1/DS3 STM mappers.

The operation of the node is controlled by a common control processor,generally illustrated in FIG. 9 by reference numeral 100.

FIG. 10 illustrates in more detail the traffic routing between the ATMtribs and ATM cell management block 70. The architecture selectedpartitions the ATM layer cell add/drop/passthrough functionality, thetrib physical medium dependent (PMD) and the transmission convergence(TC) sublayers to the ports.

ATM trib interfaces 105 and 107 perform some formatting to facilitatethe hand-off between cell control unit 102 and trib units 92, 93, whileSTS management interface 101 performs formatting to facilitate the handsbetween cell control unit 102 and STS management block 70.

In the output direction, block 161 effects POH termination and celldelineation for the ATM STS-Mc, and transmits the output cells to cellcontrol unit 102. Cell control unit 102 receives the output cells fromblock 101 and performs header inspection and address translation, ifnecessary, according to a routing table 104. Drop cells are sent to atrib interface 105 and/or 107, where they are formatted for handoff tothe trib ports. Passthrough cells recognized within the output cells,are sent back to STS management interface 101 to form the input STS-Mc.

In the input direction, control unit 102 receives add cells from tribinterface 105 and/or 107 and performs header inspection and addresstranslation, if necessary, according to routing table 106. It thenperforms usage parameter control (UPC). Block 102 performs queuemanagement of the input cell stream comprising add and passthroughcells, generates the POH and inserts the input cell stream into theinput STS-Mc for hand-off to block 70, via block 101. It is apparentthat block 102 has a much simplified design than needed for abidirectional node, which require double bandwidth, overhead processing,buffering and table routing control.

Trib ports 92 and/or 93 receive a cell structured DS1 and/or DS3 in theadd direction, and perform framing and clock recovery/synthesis. Theythen do cell delineation, and transmit the add cells to the cellmanagement block 90. For the drop direction, the drop cells receivedfrom cell management block 90 are framed into a DS-n format andtransmitted to the user.

Squelching is performed by cell control unit 102. With a BLSR, there isthe concern for misconnecting X to Y traffic under node failure or nodeisolation scenarios, due to the timeslot reuse capability. With a ULSRcarrying ATM traffic, a similar concern exists if cells reuse the sameVPI/VCI and a common node fails or is isolated. Similar to a BLSR, theATM ULSR will combat this issue by squelching traffic destined for thefailed node. Even if VPI/VCI reuse does not occur around the ATM ULSR,there is still a benefit in squelching traffic destined for a failednode. The squelching, or discarding of cells destined for a failed nodeat the point they would enter the ring, ensures bandwidth is not wastedthat could be used by other ATM traffic. Note that this has no analogyfor STM traffic on a BLSR, since all bandwidth is dedicated. The generalgoal discussed in GR-2837-CORE for removing isolated cells is enhancedand used for the ring of this invention. The specific mechanism consistsof the actions illustrated in FIG. 11 and described next for node A ofFIG. 5B.

In steps S14-S15, the STS protection controller block 115 determineswhether or not a node has failed or has been isolated whenever K-bytechanges occur. If so, the corresponding node identification (D) is foundby comparing the node IDs in the new K-bytes with the ring node map, instep S16.

Next, the ID of the lost node is passed to the add interface 107 in stepS17. Given the lost node ID, cell control unit can determine whethernode A is adding cells with VPIs/VCIs associated with the lost nodeusing routing table 106, which lists node IDs vs. terminating VPIs/VCIs.This is illustrated at step S17.

Locally added cells with VPIs/VCIs for the lost node are discarded bythe cell control unit in step S18.

With this scheme, cells which have no place to go never enter the ring.This approach differs from the discussion in GR-2837-CORE, which doesnot appear to achieve the goal of freeing up as much ring bandwidth aspossible. According to GR-2837-CORE, isolated cells are discarded onlyat the ring switching nodes, and therefore there would still be portionsof the ring carrying isolated cells from the add node(s) to theswitching nodes.

It is important to note that the ATM layer function of discardingincoming cells based on invalid VPI/VCI is a basic policing operation.Also, the SONET layer function of determining lost nodes is a basicprotection function. The added functionality in the ring of theinvention is the look-up table search of lost node ID vs. terminatingVPIs/VCIs and the coordination of the existing functions mentioned.

FIG. 12 shows an example of a ring according to the invention. Serviceaccess multiplexer (SAM) 121 receives Ethernet traffic and adapts itinto a DS1 ATM UNI. As well, SIM traffic, for example POTS signals atvoice frequencies (vf), may be input to a private branch exchange (PBX)122 to obtain a DS-1 trunk. The add/drop multiplexer (ADM) 123constitutes a node in the ring 120 of the invention. Node 123 assemblesthe STM traffic into a VT1.5 and inserts it into STM STS-1#2, whichtravels around the ring. Similarly, the DS-1 UNIs received from the SAM121 are assembled in the ATM STS-1#1.

FIG. 13A illustrates a 2F-BLSR in a configuration for homing trafficwhere add/drop is only performed on one side of each node. The ring isused to transport bidirectionally a STS-3. Each node adds/drops a DS3 inboth directions, as shown by the arrows between the nodes, and as suchthe add/drop traffic is symmetrical. It is apparent that a bidirectionalSTS-3 is available between adjacent nodes.

FIG. 13B shows an application for providing a unidirectional path for ahoming traffic with symmetrical add/drop at each node. It is apparentthat a unidirectional STS-3 is available between all the nodes, whichallows for an extra capacity of STS-3c in the opposite direction, asillustrated by the arrow in the ring, the traffic being still protectedThis extra capacity may be used for the same type of traffic, or forunidirectional broadcast, as for example for delivering TV programs.

While the invention has been described with reference to particularexample embodiments, further modifications and improvements which willoccur to those skilled in the art, may be made within the purview of theappended claims, without departing from the scope of the invention inits broader aspect.

1. A method for communicating information over a BLSR SONET lineswitched ring having a plurality of communication terminals connectedover a first and a second transmission line, the method comprising thesteps of: at each terminal, inserting an outgoing signal onto said firsttransmission line along a first direction of transmission defined fromsaid terminal towards the first adjacent terminal, inserting saidoutgoing signal onto said second transmission line along a seconddirection of transmission defined from said terminal towards a secondadjacent terminal; at each said terminal, receiving an incoming signalover said first transmission line from said first adjacent terminal,along said first direction of transmission, and from said secondadjacent terminal along said second direction of transmission; andoperating said plurality of communication terminals according to aunidirectional protection protocol upon detection of a failure conditionin said incoming signal, wherein said incoming signal comprises anincoming non-ATM STS-W and an incoming ATM STS-Mc multiplexed in anincoming STS-N, and said outgoing signal comprises an outgoing non-ATMSTS-W and an outgoing ATM STS-Mc, multiplexed in an outgoing STS-N,where M+W=N, and N, M, and W are integers indicative of the rates ofsaid respective signals, wherein said unidirectional protection protocoloperates according to a variant of a BellCore GR-1230-CORE standard. 2.A method as claimed in claim 1, wherein an incoming STM STS-W and anoutgoing STM STS-W each comprises a plurality of virtual tributaries(VT).
 3. A method as claimed in claim 1, wherein said incoming signalcomprises only said STS-Mc and said outgoing signal comprises only saidoutgoing ATM STS-Mc; and wherein said unidirectional protection protocoloperates according to a variant of a BellCore GR-1230-CORE standard,comprising unmodified assignments for all bytes of the transportoverhead (TOH) field of said incoming signal, except for a modifiedassignment of bits 0-4 of the K1 byte, wherein a first span code is usedfor a lockout of protection code, a second span code FS-S is used for asignal fail on protection code, and third span codes, and reverserequest codes are eliminated; and a modified assignment of bits 6-8 ofthe K2 byte, wherein the code “Bridges and Switched” is used for a code“Switch”.
 4. A method as claimed in claim 1, wherein said unidirectionalprotection protocol operates according to a Bellcore GR-230-COREstandard for said incoming and outgoing non-ATM STS-W and according to avariant of a BellCore GR-1400-CORE standard for said incoming andoutgoing ATM STS-Mc, said variant comprising unmodified assignments forall bytes of the transport overhead (TOH) filed of said incoming signal,except for a modified assignment of bits 0-4 of the K1 byte, wherein afirst span code is used for a lockout of protection code, a second spancode FS-S is used for a signal fail on protection code, and third spancodes (SF-S), (SD-S), and reverse request codes are eliminated; and amodified assignment of bits 6-8 of the K2 byte, wherein the code“Bridges and Switched” is used for a code “Switched”.